The pore-fluid pressure of a rock formation, which is also referred to as simply the pore pressure, is measured relative to normal pressure at the depth of the formation, in other words relative to the hydrostatic pressure of a column of water at that depth. During the drilling of a petroleum well, accurate knowledge of formation pore pressure is necessary to ensure that formation fluids do not flow into the wellbore, which can potentially cause well blow-outs. A drilling fluid, usually referred to as drilling mud or simply mud, with desired weight and rheological properties is maintained in the wellbore as the primary method for controlling formation fluid flow. A problem with the use of drilling mud, however, is that if the pressure in the wellbore resulting from the mud's weight is too high, relative to the pore pressure, the drilling rate may be decreased unnecessarily. In addition, if the pressure resulting from the mud's weight is excessively high compared to the pore pressure, that pressure may exceed the formation fraction pressure, potentially causing a loss of mud into the formation, and/or a well control problem. It is preferable therefore if the muds used in drilling wells result in an optimum range of wellbore pressure, relative to pore pressure, such that wells may be drilled safely but expediently. This is often difficult, unfortunately, because accurate pre-drill knowledge of pore pressures is not always available, especially in areas with few previously drilled wells or where the geology is complex.
More specifically, drilling operations at present generally attempt to ensure that the wellbore pressure at any given depth is about 0.5 pounds per gallon (0.5 ppg) greater than the highest estimated pore pressure at that depth. This 0.5 ppg wellbore pressure safety margin is in part required due to industry's present inability to accurately predict pore pressures in the various formations through which the drilling assembly will drill. Reducing the uncertainty in knowing the pore pressure ahead of the bit would lead to significant reductions in the cost of drilling operations, as a result, for example, of an improved ability to specify casing setting depths and of an increase in the rate at which wells are drilled. The present invention allows continuous estimation of pore pressures of formations ahead of the drilling assembly, and thereby allows drilling operations to be carried out with lower average mudweights—in other words with mud weights which are optimized for the formations to be drilled and thereby do not require use of as large a pressure margin as is current practice.
Data presently used to estimate the pore pressure profile versus depth at proposed well locations include offset well data, surface seismic data, seismic-while-drilling data, and geologic models. Pressure measurements from nearby offset wells can provide the most accurate pre-drill pressure information, but for remote locations these data are generally not available. Pore pressure estimation from surface seismic data is based on an empirical relationship between the velocity of sound waves in the formation and pore pressure, with assumptions made for the nature of the formation, in other words the type of rocks that are expected to be present (which is also referred to as formation lithology). This relationship is based on a number of different properties which are understood in industry. For example, formation velocity estimation from seismic data using normal moveout analysis techniques is well understood in the art. Equally well understood is the fact that formation velocity is a function of both the elastic moduli and the density of the rock, and that formation velocity generally increases with depth as rocks become more and more compacted. It is also understood that an increase in pore pressure with depth often coincides with a decrease in this increasing velocity trend (or even an actual decrease in velocity with depth) because the higher pore pressure is associated with less compacted rock. These combined factors allow derivation of empirical velocity-pore pressure relationships for use with seismic data.
Pore pressure predictions from seismic data analysis typically suffer from large uncertainty however. There are several contributing factors to this uncertainty, including the inherent uncertainty in the velocity models, the uncertainties in the variation of lithology compared with the data used to build the velocity-pore pressure empirical relationships, and the low vertical resolution of the seismic data. In addition, large and significant pore pressure variation can occur over vertical intervals of rock much thinner than that which seismic data can resolve.
Seismic-while-drilling (SWD) is a method for estimating formation velocity above and below the drill-bit during the drilling process. Geophones and/or hydrophones placed at the earth's surface around the well being drilled record the seismic signals produced by the drill-bit as it drills into the formation. Although the drill bit may emit frequencies across the acoustic band up to or above approximately 20 kiloHertz (20 kHz), only the frequencies in the seismic band (which will be understood to those skilled in the art as less than about 100 Hz., and more specifically less than about 80 Hz.) propagate to the surface. In addition to the seismic band signals, an acoustic signal from the drill bit also propagates along the drill string assembly to the surface. The signal to be used to determine formation velocity is detected by cross correlating the signal propagating through the earth with the signal that has propagated along the drill string. See for example the disclosure of Staron, Arens, and Gros, in U.S. Pat. No. 4,718,048 titled “Method of Instantaneous Acoustic Logging Within a Wellbore.” That signal is usually at a single frequency, typically about 50 Hz, and, using inversion processing, which is analogous to surface seismic processing, can be used to estimate the acoustic impedance and velocity of intervals below the drill bit. Pore pressure is then estimated using the same velocity-pore pressure empirical relationships used with surface seismic data.
Compared to pore pressure prediction using surface seismic methods, the main advantages of SWD are that the depth to sub-surface reflectors is better constrained and vertical resolution is improved. Unfortunately, there are also some important limitations with SWD. For example, the resolution of analytic results from SWD data is generally limited by the relatively low seismic wave frequencies. Second, poor SWD signals are received with polycrystalline diamond compact (PDC) bits, which are generally the preferred bits for drilling operations where high pore pressure is expected to be encountered. Traditional roller-cone bits provide the best SWD signals but may compromise efficient drilling operations in many areas. Third, drilling with downhole motors that rotate the bottomhole assembly while leaving the rest of the drillpipe non-rotating has become a preferred method in many areas, but that method also provides poor SWD signals. One method proposed to improve SWD in these situations, such as disclosed by Barr et. al. in U.S. Pat. No. 4,873,675 titled “Method and Apparatus for Seismic Exploration of Strata Surrounding a Borehole,” uses drilling jars, which are apparatus made to violently move the bottomhole assembly up or down on demand to free stuck pipe, as the acoustic source rather than the drill bit. The drilling jar method involves downhole detection of the reflected signal with a downhole geophone run on a cable with a side-door entry sub. Unfortunately, Barr's method is not feasible in most situations because of the need for the cable, which is disruptive to the drilling operation. Another method, disclosed by Beresford and Crowther in U.S. Pat. No. 5,798,488 titled “Acoustic Sensor,” uses a downhole acoustic transducer to both send and receive the acoustic signal. Beresford and Crowther do not disclose a method for determining formation properties however.
Seismic data is also used to guide the drilling process, for example to aid identification of potential high-pressure zones. However, seismic signal velocities are poorly correlated with high-pressure zones, and seismic data resolution is far below that needed to make decisions during drilling. Increased seismic data resolution can be achieved by employing Vertical Seismic Profiling (VSP). In VSP, geophones are lowered into the borehole so that the precise depth of the geophone is known and only the one way seismic travel times need to be measured. A major disadvantage of VSP, however, is that the drill string must be removed for VSP measurements. VSP data is therefore by necessity only taken over limited intervals.
A method and apparatus is desired which will facilitate accurate estimation of the pore pressure in rock formations before such formations have been penetrated by a drilling assembly. Preferably, this method and apparatus should not require withdrawal of the entire drill string from the borehole each time measurement data is to be acquired, and should preferably allow generally continuous, if so desired, estimation of pore pressures in the formations directly ahead of the drilling assembly. The present invention addresses these objectives.